The present invention relates to an automatic position control method for positioning a small object, for example, a pellet used when a semiconductor element is manufactured.
When manufacturing semiconductor elements such as transistors or integrated circuits, it is necessary to position a very small part accurately at a predetermined position. Such a case is, for example, the positioning operation in the die mount or wire bounding process. The prior method for such positioning is involves moving the small object manually by using a microscope. This conventional method, however, is problematic in that the positioning operation is complex and inefficient and requires a highly skilled technique. Attempt has been made to automatise that positioning operation of the conventional method, with an unsatisfactory result from difficult problems of technique. The television camera, for example, is often used as a position detector for automatic positioning. However, this method by the television camera suffers from many problems. For example, since current must constantly flow through the rustor coil the heat evolved from the coil possibly deforms the components and causes the aging change of the optical axis. Since an analogue signal is used for detecting the position of the small object to be controlled, the positioning operation is sensitive to external noise and the detected value tends to be drifted and the circuit for the conversion of the analogue into a digital signal becomes complicated and expensive. The residual image of the camera tube inevitably elongates the cycle time necessary for the position detection, resulting in inefficiency of the work.
There is known a method for using a magnified projection image of a small object so as to detect the position of an object for positioning. In this method an object to be controlled is projected onto a screen to form a magnified image of the object thereon. Solar batteries are disposed on the screen. The positions of the edges of the projected image are detected in an analogue fashion through the solar batteries' outputs. The object is moved so that the outputs of the solar batteries have respectively a predetermined value. This projection image method, however, fails to accurately position the object due to the fact that the side surfaces of the object are generally not normal to the upper and lower surfaces, the periphery of the object is relatively uneven in contour and the light reflection from the edge of the objects has a complicated nature. Further, even if the object has minor cutaway portions on the periphery providing in fact no problem, such defects are detected and in consequence, the positioning of it is inaccurate. Moreover, since the position detector by the magnified projection image uses the analogue signal obtained from the solar batteries, in addition to the above-mentioned problems of drift and noise, it is further defective in that, if dust or the like adheres to the vinyl sheet on which the semiconductor pellet is mounted as found in the die mount process of semiconductor manufacturing, the output of the solar batteries is reduced, resulting an inaccurate detection of the edge positions of the object.